1 / 33

Understanding EER Models for Database Design

Explore the Enhanced Entity-Relationship (EER) Model, its concepts like subclasses, superclasses, inheritance, and how it creates accurate database schemas for complex needs beyond traditional applications. Learn about subtypes, specializations, attribute inheritance, and more.

pringle
Download Presentation

Understanding EER Models for Database Design

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Chapter 8 The Enhanced Entity-Relationship (EER) Model

  2. The Enhanced Entity-Relationship (EER) Model • Enhanced ER (EER) model • Created to design more accurate database schemas • Reflect the data properties and constraints more precisely • More complex requirements than traditional applications

  3. Subclasses, Superclasses, and Inheritance • EER model includes all modeling concepts of the ER model • In addition, EER includes: • Subclasses and superclasses • Specialization and generalization • Category or union type • Attribute and relationshipinheritance

  4. Subclasses, Superclasses, and Inheritance (cont’d.) • Enhanced ER or EER diagrams • Diagrammatic technique for displaying these concepts in an EER schema • Subtype or subclass of an entity type • Subgroupings of entities that are meaningful • Represented explicitly because of their significance to the database application

  5. Subclasses, Superclasses, and Inheritance (cont’d.) • Terms for relationship between a superclass and any one of its subclasses • Superclass/subclass • Supertype/subtype • Class/subclass relationship • Type inheritance • Subclass entity inherits all attributes and relationships of superclass

  6. Subclasses and Superclasses (1) • An entity type may have additional meaningful subgroupings of its entities • Example: EMPLOYEE may be further grouped into: • SECRETARY, ENGINEER, TECHNICIAN, … • Based on the EMPLOYEE’s Job • MANAGER • EMPLOYEEs who are managers • SALARIED_EMPLOYEE, HOURLY_EMPLOYEE • Based on the EMPLOYEE’s method of pay • EER diagrams extend ER diagrams to represent these additional subgroupings, called subclasses or subtypes

  7. Subclasses and Superclasses (2) • Each of these subgroupings is a subset of EMPLOYEE entities • Each is called a subclass of EMPLOYEE • EMPLOYEE is the superclass for each of these subclasses • These are called superclass/subclass relationships: • EMPLOYEE/SECRETARY • EMPLOYEE/TECHNICIAN • EMPLOYEE/MANAGER • …

  8. Subclasses and Superclasses (3) • These are also called IS-A relationships • SECRETARY IS-A EMPLOYEE, TECHNICIAN IS-A EMPLOYEE, …. • Note: An entity that is member of a subclass represents the same real-world entity as some member of the superclass: • The subclass member is the same entity in a distinct specific role • An entity cannot exist in the database merely by being a member of a subclass; it must also be a member of the superclass • A member of the superclass can be optionally included as a member of any number of its subclasses

  9. Subclasses and Superclasses (4) • Examples: • A salaried employee who is also an engineer belongs to the two subclasses: • ENGINEER, and • SALARIED_EMPLOYEE • A salaried employee who is also an engineering manager belongs to the three subclasses: • MANAGER, • ENGINEER, and • SALARIED_EMPLOYEE • It is not necessary that every entity in a superclass be a member of some subclass

  10. Representing Specialization in EER Diagrams

  11. Attribute Inheritance in Superclass / Subclass Relationships • An entity that is member of a subclass inherits • All attributes of the entity as a member of the superclass • All relationships of the entity as a member of the superclass • Example: • In the previous slide, SECRETARY (as well as TECHNICIAN and ENGINEER) inherit the attributes Name, SSN, …, from EMPLOYEE • Every SECRETARY entity will have values for the inherited attributes

  12. Specialization (1) • Specialization is the process of defining a set of subclasses of a superclass • The set of subclasses is based upon some distinguishing characteristics of the entities in the superclass • Example: {SECRETARY, ENGINEER, TECHNICIAN} is a specialization of EMPLOYEE based upon job type. • May have several specializations of the same superclass

  13. Specialization (2) • Example: Another specialization of EMPLOYEE based on method of pay is {SALARIED_EMPLOYEE, HOURLY_EMPLOYEE}. • Superclass/subclass relationships and specialization can be diagrammatically represented in EER diagrams • Attributes of a subclass are called specific or local attributes. • For example, the attribute TypingSpeed of SECRETARY • The subclass can also participate in specific relationship types. • For example, a relationship BELONGS_TO of HOURLY_EMPLOYEE

  14. Specialization and Generalization • Specialization • Process of defining a set of subclasses of an entity type • Defined on the basis of some distinguishing characteristic of the entities in the superclass • Subclass can define: • Specific attributes • Specific relationship types

  15. Specialization and Generalization (cont’d.) • Certain attributes may apply to some but not all entities of the superclass • Some relationship types may be participated in only by members of the subclass

  16. Generalization • Reverse process of abstraction • Generalize into a single superclass • Original entity types are special subclasses • Generalization • Process of defining a generalized entity type from the given entity types

  17. Constraints and Characteristics of Specialization and Generalization Hierarchies • Constraints that apply to a single specialization or a single generalization • Differences between specialization/ generalization lattices and hierarchies

  18. Constraints on Specialization and Generalization (1) • If we can determine exactly those entities that will become members of each subclass by a condition, the subclasses are called predicate-defined (or condition-defined) subclasses • Condition is a constraint that determines subclass members • Display a predicate-defined subclass by writing the predicate condition next to the line attaching the subclass to its superclass

  19. Constraints on Specialization and Generalization (2) • If all subclasses in a specialization have membership condition on same attribute of the superclass, specialization is called an attribute-defined specialization • Attribute is called the defining attribute of the specialization • Example: JobType is the defining attribute of the specialization {SECRETARY, TECHNICIAN, ENGINEER} of EMPLOYEE • If no condition determines membership, the subclass is called user-defined • Membership in a subclass is determined by the database users by applying an operation to add an entity to the subclass • Membership in the subclass is specified individually for each entity in the superclass by the user

  20. Displaying an attribute-defined specialization in EER diagrams

  21. Constraints on Specialization and Generalization (3) • Two basic constraints can apply to a specialization/generalization: • Disjointness Constraint: • Completeness Constraint:

  22. Constraints on Specialization and Generalization (4) • Disjointness Constraint: • Specifies that the subclasses of the specialization must be disjoint: • an entity can be a member of at most one of the subclasses of the specialization • Specified by d in EER diagram • If not disjoint, specialization is overlapping: • that is the same entity may be a member of more than one subclass of the specialization • Specified by o in EER diagram

  23. Constraints on Specialization and Generalization (5) • Completeness Constraint: • Total specifies that every entity in the superclass must be a member of some subclass in the specialization/generalization • Shown in EER diagrams by a double line • Partial allows an entity not to belong to any of the subclasses • Shown in EER diagrams by a single line

  24. Constraints on Specialization and Generalization (6) • Hence, we have four types of specialization/generalization: • Disjoint, total • Disjoint, partial • Overlapping, total • Overlapping, partial • Note: Generalization usually is total because the superclass is derived from the subclasses.

  25. Example of disjoint partial Specialization

  26. Example of overlapping total Specialization

  27. Specialization/Generalization Hierarchies, Lattices & Shared Subclasses (1) • A subclass may itself have further subclasses specified on it • forms a hierarchy or a lattice • Hierarchy has a constraint that every subclass has only one superclass (called single inheritance); this is basically a tree structure • In a lattice, a subclass can be subclass of more than one superclass (called multiple inheritance)

  28. Shared Subclass “Engineering_Manager”

  29. Specialization/Generalization Hierarchies, Lattices & Shared Subclasses (2) • In a lattice or hierarchy, a subclass inherits attributes not only of its direct superclass, but also of all its predecessor superclasses • A subclass with more than one superclass is called a shared subclass (multiple inheritance) • Can have: • specialization hierarchies or lattices, or • generalization hierarchies or lattices, • depending on how they were derived • We just use specialization (to stand for the end result of either specialization or generalization)

  30. Specialization/Generalization Hierarchies, Lattices & Shared Subclasses (3) • In specialization, start with an entity type and then define subclasses of the entity type by successive specialization • called a top down conceptual refinement process • In generalization, start with many entity types and generalize those that have common properties • Called a bottom up conceptual synthesis process • In practice, a combination of both processes is usually employed

  31. Specialization / Generalization Lattice Example (UNIVERSITY)

More Related